Hope my terminology is correct. Some target grips have an adjustable palm rest which comes up and around the outside of the palm. I am wondering how high up the outside of the palm it can come. I like to tinker.
Depends on the grip and the pistol it's designed for. It should lay tight against the edge of your palm when you grip is as high as you can make it. And when I say, "tight" I mean that the flesh there should be compressed.
The purpose of the palmrest is to counter muzzle weight. The purpose of your grip (the thing you do with your hand, not the thing you put on your gun) is to hold the gun so that it moves as little as possible in your hand, while allowing you to break the trigger without disrupting your aim.
Ah! There's no limit to how high the palmrest itself can come. You run into rule problems when you start wrapping parts of the grip around the outside of your hand. In which case you're limited (as Ham points out) by angle at the top, and a height limitation of an inch at the bottom.
I would personally be conservative about it. There's not a lot to be gained, and you're really at the mercy of the referee and match directors at a sanctioned event.